OK, so you have a real problem there. First, let's identify the four black/white pairs you have there, and they probably come out of cables. I'm going to label these AH for Always-Hot, N for Neutral, and SH for Switched Hot.
- Cable 1: Power supply from source (AH and N)
- Cable 2: Onward power supply to the next outlet or possibly the ring circuit, which goes clear around back to the panel, since this is UK). (AH and N)
- Cable 3: The spur line to the switch, or "switch loop" (AH and SH)
- Cable 4: Switched power onward to the other lamp (SH and N)
- The lamp itself takes SH (brown) and N (black).
In your original setup, the SH wires were designated with tippex. Now your problem is figuring out what those are.
Identifying your cables
With all the wires unhooked, I would touch each one with a non-contact tester. Exactly one red will be hot - that's supply (cable 1). Because you are in the UK, you may have "Ring Circuits" and another red may also be hot. That is fine; call that Cable 2.
By the way, hot means full 230V, if you use a DVM, you may see wimpy voltages like 205V or 134V. That is phantom voltage, and is not real.
Identify the always-hot wire(s) in some way. Since the new color code is brown/blue, I would identify them with brown tape, wrapped around the wire several times, maybe even spiraled for some distance up the wire. Or better yet, brown shrink-tubing shrunk on. I would not use conventional UK non-shrink sleeve for this, because the whole point is, it needs to not fall off!
Now, turn off your circuit breaker/pull the fuse.
I hope you can get back behind the rose and see how the cables are actually grouped. The 1 or 2 always-hot wires you just identified, will come from 1 or 2 cables. You need to identify their partner neutral wires - the black neutrals that are in the same cable. I would mark those with blue tape to designate actual-neutral.
Now, it's experimentation time -- it's not good to experiment generally, but if you do it in a controlled way, it can be OK. And presumably we have one of those nice British 3/3/2 splice blocks to work with!
Place the known supply neutral on the end 3-block. Place the known always-hot on the other 3-block. Connect the lamp's (or any lamp's) hot and neutral to that same 3-block. Go turn the breaker on and see if the lamp lights up. That will confirm you've identified the power cable. We won't leave it this way.
If you identified a second always-hot cable earlier, unhook the first cable, and hook this one up in its place, and power up the breaker again, and see if the lamp lights up on this cable also. That would indicate a ring circuit.
So, once you've confirmed they are power cable(s), permanently hook their neutral(s) to the end 3-block and their hot(s) to the other 3-block. We will not be touching that again.
Test each cable, one at a time
Next, with the switch OFF, try each of the other cables, one at a time. Hook its red on the middle/red 3-block, and its black to the end/black/neutral 3-block. One of three things will happen.
- Nothing (in which case this is most likely the switch loop). Immediately unhook these wires; they are in the wrong place.
- Light switches in other rooms that didn't work, suddenly now work. Remember: DO NOT turn on the switch in this room!
- If this is the case, remark this cable's red wire with brown tape (it is always-hot) and its black wire with blue tape (it is neutral), and put it back on the appropriate 3-blocks permanently. It will never be removed again. And this is a good breaking point, because at this point everything should work except this room.
- The other lamp in this room lights continuously (no way to turn it off; DO NOT touch the switch!!)
- If so, its red is switched-hot. Shut down the breaker, and mark its black wire with blue tape. It is a neutral. This wire gets put (left?) on the end 3-block and lives there permanently. The red wire gets put on the 2-block.
Finish the hookup
So after you iterate with each red/black pair, you should now have 3 blacks-marked-blue on a 3-block, 2 reds-marked-brown on a 3-block, and and red on a 2-block, as well as a loose cable not yet allocated, and the lamp of course.
What is left? The loose cable is the switch. Mark the red wire with brown tape. It permanently joins the always-hot 3-block with the other remarked browns. Mark its black wire with red tape or a proper UK sleeve for marking switch loops. It goes to the 2-block with the other red wire. We're almost home!
On the lamp, the leads are probably blue and brown. Mark the brown wire red, to signify that it's a switched-hot (you don't want this light on 24x7, right?) Its blue goes onto the neutral 3-block, its brown->red goes onto the 2-block. Note that these blocks are full; you'll have to double-tap a block point.